A film demonstrating the flammability of different types of film stock.
During the 1950s, nitrate film was replaced by acetate 'safety film', which was far less flammable (acetate was subsequently replaced by poyester in the 1980s - see the separate page on polyester later in this tour). The video above contains a demonstration of the different levels of flammability of the three major film formats.
With the replacement of nitrate with acetate, many of the safety requirements for nitrate prints no longer applied. The limits on the length of projector reels was relaxed, meaning that longer running times between change-overs were available.
One result of this was that the carbon arc lamps that had been used to light the screens since the birth of cinema could no longer be used. This was because the carbon rods that served as the light source had to be replaced every 20-40 minutes. With 60 minute+ acetate reels the carbon rods would burn out before the end of the reel.
As a result, long-life xenon bulbs were introudiced and have been used ever since. In this audio clip Neil Thompson discusses the change from carbon arc lamps to xenon lamps.